It is well known that dual polarized antennas may be used for communication via channels with different fading statistics, thereby better utilizing the available spectrum in a wireless network.
It is also known that MIMO systems may increase the data rate in wireless communication systems where the available channel bandwidth is fixed. In MIMO applications, a given data stream is split into a number of individual data streams and transmitted over a common frequency band using multiple antennas at the base-station and the user equipment. In a fading environment characterized by multi-path propagation, each transmission path will be subject to different fading characteristics, which may be estimated by means of transmitted pilot sequences or reference signals. This property is utilised in a MIMO reception system to resolve the individual data streams. In order for the MIMO system to work properly, the magnitude of the correlation p between the signals that are communicated via the channels exploiting the antenna arrangement must be sufficiently low, typically below 0.7. A common way to achieve low-correlated data streams is to use spatially separated antennas that experience different channel fading statistics. An alternative option is to transmit/receive the different data streams utilising antennas of orthogonal polarizations. Multimode antennas where each mode has a different radiation pattern is yet another technique [T. Svantesson, “Correlation and channel capacity of MIMO systems employing multimode antennas”, IEEE Trans. on Vehicular Technology, Vol. VT-51, pp. 1304-1312, November 2002].
FIG. 1 shows three antenna arrays M, M′ and M″ being arranged with a distance, d, between one another, all radiating elements having the same polarization. FIG. 2 and FIG. 3 show other more compact configurations wherein two arrays M and M′ are interleaved and arranged on a line but where the elements of one array M have an orthogonal polarization in relation to the elements of the other antenna array M′. In FIG. 4, a fourth configuration is shown having two separated antenna arrays M and M′, wherein all radiating elements of a particular array are oriented in the same direction and hence have the same polarization, but where the elements of the two arrays are orthogonally polarized in relation to one another. In a fifth, FIG. 5, and a sixth, FIG. 6, configuration, the two antenna arrays are co-located employing dual-polarized radiating elements with common phase centers for the two polarizations. Other configurations include a plurality of single or dual polarized antenna arrays or a combination thereof placed side by side or aligned above each other.
For the configuration examples in FIGS. 1-6, excitation weighting networks shown in FIG. 7 may be provided that have a magnitude weight, A, and a phase or delay weight, α, for each radiating element, the magnitude weights, and delay weights also being denoted excitation weights or excitation means. It is understood that the delay weights can be implemented as true time delay weights or as phase weights between 0 and 360 degrees or a combination thereof. The former implementation with only true time delay weights gives a more broadband system compared to only a phase weight implementation. By assigning various values to the individual excitation weights, various effects may be accomplished such as to direct the main beam of the antenna at a desired angle θ with regard to the antenna array, to control the side-lobe level, and to shape the radiation pattern. In several prior art antenna array systems, the magnitude weights and the delay weights of N radiating elements are chosen such that An=A′n and αn=α′n, where n is from 1 to N, that is the respective antenna arrays are identical with regard to the excitation means of the same respective position in the arrays for diversity transmission or reception.
U.S. Pat. No. 6,282,434 shows a method for providing quality improvement by applying different antenna radiation amplitude patterns by mechanically or electronically down-tilting the receive antenna array at a different angle in relation to the transmit antenna array. The electronically down-tilted beam is accomplished by applying only different phase weights to the radiating elements of the receive array such that a linear progressive phase shift between the radiating elements is achieved.
Many state-of-the-art base-station antenna installations make use of spaced apart antenna arrays as shown in FIG. 1. Some installations are dual-polarized as shown in FIGS. 2-6. Such antenna arrays may infer a correlation between the received signals due to radiation pattern de-polarization. The correlation between signals received in the dual-polarized beams is usually very low within the angular region of the main beam. However, in the side-lobe region, the correlation may increase, especially for the dual-polarized or closely spaced antenna configurations shown in FIGS. 1-6. This may be disadvantageous for high data rate capable mobile terminals that are located close to the base-station and therefore communicating via the side-lobe angular region in the base-station antenna radiation pattern.
One problem associated with prior art antenna arrangements is that the conditions for transmit and receive diversity applications or MIMO applications are not sufficiently fulfilled.
Another problem associated with known antenna systems is that close to a base-station there may be service areas with reduced field strength due to nulls in the side-lobe region of the antenna radiation amplitude pattern.